A telescopic actuator is known, in particular from document FR 2 895 483, that comprises a cylinder in which a main rod is mounted to slide telescopically along a sliding axis between a retracted position and an extended position, the telescopic actuator including an auxiliary rod mounted to slide telescopically in the main rod along said sliding axis between a retracted position and an extended position, the actuator including controlled retaining means for retaining the auxiliary rod in the retracted position inside the main rod. Thus, so long as the main rod slides properly in the cylinder, the auxiliary rod remains blocked in the main rod, and the actuator operates like a conventional actuator. If the main rod becomes blocked, it then suffices to control the retaining means so that they release the auxiliary rod, such that the actuator is again free to lengthen and it no longer blocks the movement of the mechanical system in which it is included.
The actuator also includes brake means for braking sliding of the auxiliary rod in the main rod in the extension direction.
Such an actuator is described below with reference to FIG. 1, which is a diagrammatic section view of an actuator of the above-specified type, and is therefore known in itself. The actuator comprises a cylinder 1 defining a cylindrical cavity closed by an end wall 12 carrying a lug 13. A main rod 2 is mounted in the cavity to slide along a sliding axis X and to project to a greater or lesser extent from the cylinder 1 through an open end of the cylinder that constitutes a bearing.
The actuator includes control means for controlling the sliding of the main rod 2 relative to the cylinder 1. These sliding control means comprise:                a key 3 secured to the main rod 2 and co-operating with a groove 4 formed in the wall of the cavity of the cylinder 1 to prevent the main rod 2 from turning relative to the cylinder 1;        an electric drive motor 5 comprising a stator 6 mounted stationary in the cylinder 1, and a rotor 7 mounted to rotate inside the cylinder 1 about the axis X;        a lead screw 9 extending along the axis X and mounted to rotate within the cylinder 1 about the axis X, being driven in rotation by the rotor 7 via a coupler 8. The lead screw 9 is held axially relative to the cylinder 1 by means of a double-acting axial abutment 11; and        a complementary nut 10 formed in an end wall of the main rod 2 through which the lead screw 9 passes so as to establish a reversible helical connection between the lead screw 9 and the main rod 2.        
Thus, rotation of the rotor 7 drives rotation of the lead screw 9 which drives axial displacement of the main rod 2.
The main rod 2 is hollow and defines a cavity in which an auxiliary rod 20 is mounted to slide inside the main rod 2 along the axis X and to project through an open end thereof that forms a bearing.
the actuator further includes controlled retaining means for retaining the auxiliary rod 20 in its retracted position inside the main rod 2. These retaining means comprise:                a catch 30 extending inside the main rod 2;        a step 31 provided at the end of the auxiliary rod 20 and adapted to co-operate with the catch 30; and        a locking sleeve 32 that is axially displaceable by an unlocking motor 33 against a return spring 34 from a locking position (shown in this figure) in which it covers the catch 30 so as to prevent it from expanding radially, and a release position in which it leaves the catch 30 free to expand radially when the step 31 passes therethrough. For this purpose, the unlocking motor 33 comprises a stator 35 secured to the main rod 2, and a rotor 36 that co-operates with a complementary thread of the main rod 2 to constitute a reversible helical connection. Rotation of the rotor 36 thus causes it to move axially. The locking sleeve 32 is carried at the end of the rotor 36 and is mounted thereto by means of rollers so as to be free to rotate.        
The actuator includes controlled retaining means for retaining the auxiliary rod 20 in its retracted position inside the main rod 2. These retaining means comprise:                a catch 30 extending inside the main rod 2;        a step 31 provided at the end of the auxiliary rod 20 and adapted to co-operate with the catch 30; and        a locking sleeve 32 that is axially displaceable by an unlocking motor 33 against a return spring 34 from a locking position (shown in this figure) in which it covers the catch 30 so as to prevent it from expanding radially, and a release position in which it leaves the catch 30 free to expand radially when the step 31 passes therethrough. For this purpose, the unlocking motor 33 comprises a stator 35 secured to the main rod 2, and a rotor 36 that co-operates with a complementary thread of the main rod 2 to constitute a reversible helical connection. Rotation of the rotor 36 thus causes it to move axially. The locking sleeve 32 is carried at the end of the rotor 36 and is mounted thereto by means of rollers so as to be free to rotate.        
The actuator includes brake means for braking sliding of the auxiliary rod 20 in the main rod 2 in the direction for extending the auxiliary rod 20. These brake means which also form a bearing 21 for guiding the auxiliary rod 20 in the main rod 2 comprise:                an externally threaded driver 22 that co-operates with internal tapping of the main rod 2 so as to constitute a reversible helical connection between the driver 22 and the main rod 2;        a bushing 23 mounted to rotate on the auxiliary rod 20 by means of rollers, the driver 22 itself being mounted to rotate on the bushing 23 by means of rollers, the driver 22 being held captive axially between the bushing 23 and the rollers bearing against an abutment 24 of the auxiliary rod 20;        a ratchet-type freewheel 25 between the driver 22 and the bushing 23 which, when the auxiliary rod 20 is extended from the main rod 2 and the driver 22 turns because of the helical connection with the main rod 2, causes the bushing 23 to rotate. In contrast, when the auxiliary rod 20 returns into the main rod 2, the driver 22 turns but does not drive the bushing 23 in rotation; and        a friction washer 26 carried by the auxiliary rod 20 facing a free face of the bushing 23 and adapted to generate friction against the bushing 23 when it turns by being pressed against the friction washer 23 under external drive pulling on the auxiliary rod 20;        
It should be observed that the auxiliary rod 20 is guided axially firstly by the open end of the main rod 2 and secondly by the bearing 21. The main rod 2 is guided axially firstly by the open end of the cylinder 1 and secondly by co-operation between the nut 10 and the lead screw 9. Finally, the lead screw 9 is guided axially at one end by a ball joint 14 inserted in the double-acting abutment 11, and at the other end by a sliding ball joint 15 which is mounted to slide inside the auxiliary rod 20. By means of these dispositions, there is no need to fit the main rod 2 with a sliding bearing inside the cylinder 1.